Control system



Aug. 13, 1935. HLE 2,011,371

0011111011 SYSTEM Filed Oct. 11, 1935 lnventor: F'l-anc i s Moh l e1;

L 5. SLAM/2M bJ l tmtorneg.

Patented Aug. 13, 1935 UNITED STATES PATENT OFFICE new York Applied!October 11, 1933, Serb-I No. 3,139

5 Claims. (Cl. 8.42)

This invention relates to control systems, more particularly tosystems-for controlling the operation oi apparatus in which a strip ofmaterial is supplied from a reel to an operating element, such as a pairof mill rolls, and it has for an ob- Ject the provision of a simple,reliable and improved'system of this character.

More specifically, the invention relates to control systems for variablespeed apparatus operating upon a strip' of material and a further objectof the invention is the provision oi. means for maintainingsubstantially constant tension in the strip between the supply reel andthe operating element irrespective of the strip speed.

In carrying the invention into effect in one form thereof, an operatingelement unwinds a strip 01' material from a supply reel from which isdriven a dynamo-electric machine supplying current to a dynamic brakingresistance, and means are provided for varying the dynamic brakingresistance in accordance with the speed of the strip.

The invention is illustrated in one form thereof as applied to a coldstrip steel rolling mill in which the strip is unwound from a reel andpassed through rolls for reducing its gage. For a better and morecomplete understanding of the invention reference shouldnow be had tothe following specification and to the accompanying drawing, the singlefigure of which is a simple diagrammatical representation 01' anembodiment oi the invention.

Referringnow to the drawing, a strip of materlal it such for example ascold rolled steel is being unwound from the supply reel H by means ofmill rolls l2 through which the strip passes in the direction of thearrow. In passing through the rolls l2 the gage oi the strip I0 isreduced by a predetermined amount and the strip emerging from the millrolls may be either passed to a succeeding set of mill rolls or in thecase of a single stand reversing mill, the strip is wound upon asuitable take-up reel The mill rolls i2 are driven by any suitabledriving means shown vfor the purposes of illustration as a directcurrent motor II of the M Justable speed type, the armature of which issupplied with current from a suitable supply Source illustrated as avariable voltage generator it. As shown, the generator I4 is driven-at aspeed, which is preferably substantially constant, by any suitabledriving means such for example as that represented by the alternatingcurrent motor II which in turn is supplied from a suitable source ofalternating current represented in the drawing by the three supply linesIt to which the terminals of the motor I! are connected as illustrated.The generator I4 is provided with a field winding I] which is suppliedwith direct current from a suitable source such for example as thatrepresented bythe two supply lines I8, 20 to which the terminals of thefield windings I! are connected with the variable resistance 2iincluded'in the circuit. Suitable switching means illustrated as amanually operated switch 22 is provided for completing a loop circuitbetween the armature oi the generator I and the armature oi. the millmotor l3. This connection is known in the art as a Ward Leonardconnection and it has the advantage that the speed of the motor i2 canbe varied over a wide range of speed by varying the rheostat 2| in thefield oi the generator to vary its voltage. By means of this WardLeonard connection the speed of the mill driving motor ll may be variedover a range as great as The mill motor I3 is provided with the directcurrent field winding 23 supplied from a suitable source of directcurrent such as that represented in the drawing by the supply line 24,25 to which the field winding is connected with a variable resistance 26included in the circuit. The supply lines 24, 25 may be and preferablyare the same as the supply lines ll, 2. to which the field winding ll ofthe supply generator is connected.

In the art of rolling cold strip steel as well as in many other alliedarts, it is important to maintain a substantially constant tension inthe strip between the unwinding reel and the mill rolls. For thispurpose there is provided a dynamo electric machine 21, the drive shaftof which is connected either directly or through suitable reductiongearing to the roll ll so that the dynamo electric machine is driven ata speed such that its generated voltage is proportional to the speed ofthe reel. This dynamo-electric machine is provided with a direct currentfield winding 28 which is supplied from a suitable source of directcurrent represented by the supply lines 2!, II which may be andpreferably are the same as the supply lines It, 20 to which the field oithe generator I4 is connected. As shown, the armature member of thedynamo-electric machine 21 is connected in circuit with a dynamicbraking resistance 3! and suitable switching means illustrated as amanually operated switch 32 are provided for completing and interruptingthis dynamic braking circult. machine 21 isdrivenbythereel Ii asthestripis unwound therefrom, current is supplied to the Since the armature oithe dynamo electric i the form of heat.

Preferably the strip I is unwound from the reel II at a substantiallyconstant peripheral speed. As the diameter of the strip roll decreases,the spec-:1 of rotation of the reel II and the dynamo-electric machine21 increases proportionately. Consequently if the field excitation ofthe dynamo-electric machine 21 remains constant the current supplied tothe dynamic braking resist ance 3| would increase in proportion to theincrease in speed of rotation of the dynamo-electric machine and thisincrease in current would result in-the increase in tension in the stripl0 between the reel H and the rolls |2. However, variation in thetension of the strip is extremely undesirable and for this reason means33 are provided for maintaining the current supplied to the dynamicbraking resistance 3| substantially constant. The means33 areillustrated as comprising a constant current regulator of any wellknowncommercial type. Briefly this regulator comprises a resistance 34included in the circuit with the field winding 28 of the dynamo electricmachine 2'! anda pair of vibratory contacts 35 forrapidly opening andclosing a short circuit about the resistance 34. The lower contact 35 isstationary whilst the upper contact is mounted on a lever 36, which asshown, is arranged for limited rotation about a pivot point 31. To theopposite end of the lever 31 is secured the core 38 which is arrangedwithin the turns of a solenoid 40 so as to be attracted in a downwardlydirection in response to a predetermined energization of the solenoid.As shown, a resistance 4| is included in circuit between the armature ofthe dynamo-electric machine 21 and the dynamic braking resistance 3| andthe terminals of the solenoid 40 are connected across the resistance 4|so that the energization of the solenoid 40 is proportional to thecurrent flowing in the dynamic braking circuit. A va iable resistance 42is included in the cireuitof the operating coil 40 for the purpose ofchanging the setting of the regu lator. One end of a spring 43 issecured to the end of the lever 36 so that its pull is balanced againstthe pull of the operating coil 40. When the contacts 35 are open thefull amount of the resistance 34 is in circuit with the field winding 28thus weakening the excitation of the dynamoelectric machine 36 anddecreasing its generated voltage as well as the current supplied to thedynamic braking resistance 3|. A decrease in current weakens theexcitation of the operating coil 40 thus allowing the spring 43 to closethe contacts 35 and short circuit the resistance 34. As a result, byshort circuiting the resistance 34 the excitation of the field 28 isincreased thereby increasing the voltage and current flowing in thedynamic braking circuit. This increase in current strengthens theexcitation of the operating coil 40 so that its pull overpowers the pullof the spring 43 and opens the contacts 35 thereby removing the shortcircuit from about the resistance 34. This operation is repeated at sucha rapid rate that the variations in the, current in the dynamic brakingcircuit are extremely minute and the current flowing in this circuit ismaintained substantially constant at a value dependent upon the ratio ofthe total time the contacts 35 are open to the total time that thecontacts are closed.

The energy expended in unwinding the strip ID from the reel H isproportional to the speed of the strip and the energy input to the reelmust, except for the losses in the dynamo electric machine 21, bedissipated in the form of heat in the dynamic braking resistance 3|. -Itwill be seen that if the speed of the strip I0 is varied, the energyinput to the reel II also varies. However, the regulator 33 maintainsthe current in the dynamic braking circuit substantially constant andconsequently, if the dynamic braking resistance 3| remains constant forvarying speeds of the strip 0, the tension of the strip l0 would have tovary. This is more clearly seen from the following equations:

(l) H. P. input to reel=H. P. output of dynamo-electric machine 21.

The above equation may be rewritten in the following form:

2) TXS 33000 =KI R in which:

From an inspection of Equation (2) it is readily seen that if the speedof the strip varies and the current flowing in the dynamic circuit ismaintained constant by the regulator 33 that the value of the dynamicbraking resistance will have to vary in proportion to the speed of thestrip if the tension is to be maintained constant as desired. For thispurpose,'the movable contact arm 3| a of the dynamic braking resistance3| is mechanically connected by means of a shaft or other suitableconnection to the movable contact members 2|; and 26a of the resistances2| and 26 and is schematically indicated in the drawing. Suitable meansillustrated as a manually operated hand-wheel 44 are provided on theshaft of the movable contact members 2|a, 26a and 3h. so that when thehand-wheel is operated to adjust the speed of the mill, the ohmic valueof the dynamic braking resistance 3| is varied in accordance with thespeed of the strip.

When the movable contact arm 26a. is in its lowermost position, theresistance 26 is short circuited so that the excitation of the field 23of the mill motor is maximum. With the contact arm 2 It. in itslowermostposition the full amount of the resistance 2| is in circuitwith the field winding I! of the supply generator |4 so that the fieldexcitation of the generator and the generated voltage thereof areminimum. In the lowermost position of the contact arm 3|; the resistance3| is completely short circuited. I

It will be observed that as the hand-wheel 44 is rotated in a clockwisedirection to adjust the speed of the mill motor l3 to any desired value,the field of the generator I4 is first strengthened by graduallyshort-eircuiting the resistance 2| and when full voltage is obtained atthe terminals of the generator l4, the field of the mill motor I3 isweakened by gradually inserting the resistance 26 in the field circuitthereof.

In operation, the hand-wheel 44 is adjusted to a position such that thespeed of the mill motor is at a desired value corresponding to thatposition of the hand-wheel. Assuming that the hand-wheel 44 is turned ina clockwise direction to increase the speed of the mill and the striplo,

the energy input to the reel II will of course, in-

for

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